Known in the art is a compression ignition type internal combustion engine which arranges a particulate filter for trapping particulate matter in the exhaust gas in an exhaust passage. By doing this, it is possible to suppress the amount of particulate matter which is exhausted into the atmosphere.
In this regard, as the engine operating time becomes longer, the amount of particulate matter on the particulate filter gradually becomes greater and the pressure loss of the particulate filter gradually becomes larger. As a result, the engine output is liable to fall.
Therefore, in general, PM removal processing, which burns the trapped particulate matter so as to remove it from the particulate filter, is temporarily performed. In this PM removal processing, the particulate filter is maintained in an oxidizing atmosphere while the temperature of the particulate filter is raised to the PM removal temperature, for example 600° C.
To efficiently perform the PM removal processing, various improvements have been proposed. For example, to lower the PM removal temperature, a particulate filter is known which carries on its surface a catalyst which is comprised of a solid super strong acid which carries an active metal (see PLT 1). Further, to efficiently remove particulate matter from the particulate filter, a particulate filter which holds a catalyst having a precious metal and a solid super strong acid on its surface is known (see PLT 2). Furthermore, known in the art is a particulate filter where, downstream of a wall flow type particulate filter on which a catalyst is carried for removing the particulate matter, a cracking catalyst is arranged for cracking the unburned hydrocarbons in the exhaust gas to produce hydrocarbons of smaller numbers of carbon atoms and where the carrier of the cracking catalyst is formed from a solid acid having a Hammett acidity function H0 of −0.56 to −12 (see PLT 3). Note that the acid strength of the super strong acid is higher than the acid strength of sulfuric acid, while the Hammett acidity function H0 of the super strong acid is −12 or less.